Description

Observers are PHP callbacks. That means you may either pass a function name as a string or
an array containing an object or class and a method to call.

The callback is used as a signature for the observer, which allows you to remove it
by passing the exact same parameters to removeObserver().

Parameter

mixed $callback

Callback to notity, may either be a string containing the name of a global function
or an array containing class or object and the name of the method to call.

string $nName = EVENT_DISPATCHER_GLOBAL

Acts as a filter: notify the observer only if the notification name matches the name
passed in this parameter. Use EVENT_DISPATCHER_GLOBAL if the
observer should be notified regardles of the notification name.

string $class = ''

Acts as a filter: notify the observer only if the sender of the notification matches
the class passed in this parameter.